1. Field of the Invention
The present invention relates to a compound which is dispersed in a thermoelectric conversion material matrix as a phonon scattering material, a nanocomposite thermoelectric material including this compound, and a method of producing the same.
2. Description of Related Art
Recently, in order to reduce carbon dioxide emission in regard to global warming, techniques of reducing the ratio of energy obtained from fossil fuel have increasingly attracted attention. For example, one of the techniques is a technique of using a thermoelectric conversion material which can directly convert unused waste heat energy into electrical energy. The thermoelectric conversion material refers to a material which can directly convert heat into electrical energy without the necessity of a two-step process unlike thermal power generation, the two-step process including: a step of converting heat into kinetic energy; and a step of converting the kinetic energy into electrical energy.
The conversion from heat into electrical energy is performed using a difference in temperature between opposite ends of a bulk body which is formed from the thermoelectric conversion material. A phenomenon in which voltage is generated due to this difference in temperature was discovered by Seebeck and thus is called the Seebeck effect.
The performance of the thermoelectric conversion material is represented by a performance index ZT obtained from the following equation.ZT=α2σT/κ(=Pf·T/κ)
In the equation, α represents the Seebeck coefficient of the thermoelectric conversion material, σ represents the electrical conductivity of the thermoelectric conversion material, and κ represents the thermal conductivity of the thermoelectric conversion material. The term α2σ is collectively referred to as an output factor Pf. Z has a dimension that is an inverse of temperature. ZT, which is obtained by multiplying this performance index Z by an absolute temperature T, is a dimensionless value. ZT is referred to as “dimensionless performance index” and is used as an index indicating the performance of the thermoelectric conversion material.
In order for the thermoelectric conversion material to be widely used, this performance is required to be further improved. For the improvement of the performance of the thermoelectric conversion material, as clearly seen from the above-described equation, it is necessary to increase the Seebeck coefficient α and the electrical conductivity σ and to reduce the thermal conductivity κ.
However, it is difficult to improve all the properties at the same time, and many attempts have been made to improve any one of the properties of the thermoelectric conversion material.
For example, Japanese Patent Application Publication No. 2010-114419 (JP 2010-114419 A) discloses a technique of reducing the thermal conductivity by dispersing nanoparticles of ceramic or the like having an average particle size of 1 nm to 100 nm in a thermoelectric conversion material matrix as phonon-scattering particles so as to scatter phonons which are one of the factors for thermal conduction.
In the above-described thermoelectric conversion material, phonons are scattered at interfaces between the phonon-scattering particles. However, since the phonon-scattering particles are in the form of a particle, the phonon-scattering interfacial area is insufficient.